drm/i915/gvt: cleanup debugfs scan_nonprivbb

Remove extra chatty message for debugfs scan_nonprivbb which is used
to enable scan for non privileged batch on specific engine. Just write
target i915 engine mask instead.

Cc: Ding Zhuocheng <zhuocheng.ding@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200304032307.2983-1-zhenyuw@linux.intel.com